The functional role of a particular respiratory muscle depends on the coupling of mechanical to neural events. Because the respiratory pattern can be easily adjusted, it has been suggested that respiration can be regulated to optimize the efficiency of the respiratory muscles. In situ, the operating length of a skeletal muscle with respect to its optimal force producing length (force length characteristics) is governed by active tension and occurs at a length where the muscle is electrically active in vivo. Thus, in adult animals, adaptations take place in muscle length such that length and activation will eventually be synchronized for optimal output. Changes in body posture profoundly influence the operational length of the respiratory muscles. As posture is changed to held up (akin to sitting in awake animals), the diaphragm shortens, and if uncompensated, would result in a reduction in tidal volume due to length-tension considerations. However, the adoption of the head up posture is associated with a considerable phasic recruitment of the expiratory muscles (displaced to more favorable operating lengths), their mechanical contribution being 65% of tidal volume in this posture. Thus, head up dogs with ineffective diaphragms compensate by recruiting more effective abdominal muscles. The link between the mechanical properties of the respiratory muscles and their neural control has not been investigated. Our hypothesis is that recruitment of the respiratory muscles is related to their position with respect to their optimal length. The goals of the proposed work are to investigate the relationship between the mechanical efficiency of the respiratory muscles and their neural control in normal adult dogs. Moreover, to study the relationship between both parameters during a transitional state, the strategy of breathing is neonatal and emphysematous animals (i.e., both representing conditions where adaptations occur in operating length) will also be investigated. These studies should establish a new link between respiratory muscle mechanics and the control of breathing.